This invention relates to the partial oxidation of hydrogen sulphide, particularly the partial oxidation of hydrogen sulphide in a gas stream containing at least one aromatic hydrocarbon impurity.
Although aliphatic hydrocarbon impurities are easy to oxidise to carbon dioxide and water vapour in the combustion furnace or furnaces of a Claus process for the treatment of a feed gas containing hydrogen sulphide and are commonly encountered in such a process, aromatic impurities are not so simply oxidised. Benzene, toluene, xylene and ethyl benzene are common contaminants in Claus plant feed gases originating from some sources of natural gas or gas associated with oil deposits, the latter often being referred to as ‘associated gas’. Difficulty arises in effectively destroying such aromatic hydrocarbon contaminants in a Claus process. Failure to remove the aromatic hydrocarbon impurities effectively leads to carbon and hydrocarbon contamination of the catalyst beds of the Claus plant with subsequent loss of catalytic activity and therefore to a reduced sulphur conversion, a reduced catalyst life, higher maintenance costs, and a need to change the catalyst on a regular basis.
U.S. Pat. No. 4,933,163 discloses that the feed gas to a Claus process can include hydrocarbons, but dose not identify them.
One known way of handling the problem of aromatic hydrocarbon impurities is to preheat the feed gas and the air, which supports combustion in the furnace of the Claus plant to a temperature in the range of 150 to 500° C. Preheating from outside the Claus plant is thermally inefficient and requires the use of direct-fired heaters or gas-to-gas heat exchangers. If the heat required for preheat is taken from the Claus plant itself, less steam is available for amine stripping in the refinery where the Claus plant is located and difficulties can arise when starting up the Claus plant.
Moreover, preheating is energy intensive and has a significant effect on the cost and complexity of the Claus plant. Further, if a preheating system is retrofitted to an existing burner/furnace configuration, the increase in gas volume to the burner may lead to consequential operational problems concerned with mixing and pressure drop.
In the light of these problems, the conventional method of dealing with aromatic hydrocarbon contaminants in a feed to a Claus plant is to add a considerable volume of aliphatic hydrocarbon gas, normally methane, to the Claus feed. However, such a measure significantly increases the size and cost of the Claus unit. If the added methane is not effectively burnt it too can add to problems with carbon deposition. The reducing nature of the gas compositions in the furnace also tends to result in higher carbon monoxide concentrations as a result of incomplete combustion of methane or other aliphatic hydrocarbon added to the feed gas. In addition high levels of carbon disulphide and carbonyl sulphide typically result from the combustion of methane and these impurities also tend to have a deleterious effect on the operation of the plant.
Indeed, particularly if the hydrogen sulphide content of the feed gas to the Claus plant is low, i.e. less than 40% by volume, an approach that is sometimes necessary is to upgrade the gas processing facilities so as to form a feed gas that has an enhanced hydrogen sulphide content. One option is to add an acid gas enrichment plant, but this is an expensive solution. Another option is to substitute in the absorption towers of an existing facility a more selective amine for the one previously used to absorb hydrogen sulphide. Such an option can lead however to an increased carbon dioxide level in the natural gas product of the facility. Such an increase in the carbon dioxide content of the natural gas product may however be commercially unacceptable.
It is therefore an aim of the present invention to provide a simpler but effective method of ensuring the treatment of aromatic hydrocarbon contaminants in a feed gas to a plant for the partial oxidation of hydrogen sulphide to sulphur.